Process for producing acrylic rubber by copolymerizing acrylic ester and malonic acid derivative having active methylene group

ABSTRACT

A process for producing acrylic rubber vulcanizable with curing agents in e thiuram series is provided. The process includes the copolymerization of an acrylic ester or esters with a malonic acid derivative having an active methylene group of the formula ##STR1## wherein R 1  represents vinyl, allyl or methallyl, and X represents cyano or COOR 2  in which R 2  represents methyl, ethyl or propyl. The acrylic rubber thus produced shows better physical properties than the conventional one.

This is a continuation of application Ser. No. 792,391, filed Apr. 29, 1977, now abandoned.

The present invention relates to a process for producing vulcanizable acrylic rubber.

Conventional acrylic rubbers are stable to heat and oilproof because of high polarity of their ester structure, but are unvulcanizable with sulfur because they do not have any unsaturated groups or double bonds in their polymer backbone. Therefore, the conventional process for producing acrylic rubbers is by copolymerizing acrylic esters with a suitable amount of a cross-linking monomer which reacts with a vulcanizing agent such as soaps, ethyltetramines and tetraethylpentamines, and curing the copolymer with such a vulcanizing agent. Such monomers include those which contain halogen such as β-chloroethyl vinyl ether and vinyl chloroacetate, and those which contain an epoxy group such as allyl glycidyl ether, glycidyl acrylate and glycidyl methacrylate.

Such conventional acrylic rubbers are, however, liable to scorch during storage and have low resistance to cold and poor processibility. Particularly, the acrylic rubbers produced by use of liquid polyamine as a vulcanizing agent have undue adhesiveness to a mixing roll, poor bin stability, high corrosiveness, and offensive odor and toxicity coming from the amine.

It is an object of this invention to provide a process of producing vulcanizable acrylic rubber which does not have such disadvantages.

According to the present invention, an acrylic ester or esters are copolymerized with a malonic acid derivative having an active methylene group to produce acrylic rubber which can be cured with vulcanizing agents in the thiuram series for cross-linking.

FIG. 1 is a graph showing the vulcanization curves for an acrylic rubber produced according to this invention and for the conventional acrylic rubber.

FIG. 2 is a similar graph for the acrylic rubbers prepared in Example 9.

The malonic acid derivative having an active methylene group utilized in the present invention has the following general formula: ##STR2## wherein R₁ represents vinyl, allyl or methallyl group and X represents COOR₂ or cyano group.

First, if X represents COOR₂ in the formula (1), the malonic acid derivative has the following general formula: ##STR3## wherein R₂ represents methyl, ethyl or propyl group. Thus, the derivatives are malonic acids with an active methylene group having one of two acid radicals esterified with an unsaturated alcohol such as allyl alcohol, and having the other acid radical esterified with a saturated alcohol. Such derivatives include allyl ethyl malonate and allyl methyl malonate, for example.

A process for producing the former will be described by way of example. A mixture of 1 mole of ethyl cyanoacetate, 1 mole of sulfuric acid and 1 mole of water is kept at 80° C. or lower for about four hours under stirring. 1.5 mole of allyl alcohol is added, the mixture being allowed to react with slow stirring at room temperature for about 72 hours. The mixture is then rinsed, dehydrated and distilled under reduced pressure. During distillation, ethyl cyanoacetate distills off first and allyl ethyl malonate distills off last. In this process, if allyl cyanoacetate and ethyl alcohol are used as the starting materials the reaction product contains only allyl cyanoacetate and allyl ethyl malonate, containing no ethyl cyanoacetate which has a bad effect on cross-linking.

Next, if X represents cyano group in the formula (1), the malonic acid derivative has the following general formula: ##STR4## wherein R₁ represents vinyl, allyl or methallyl group. The malonic acid derivatives include esters of cyanoacetic acid (that is, malonic acid mononitrile) having an active methylene group with an unsaturated alcohol, such as allyl cyanoacetate or methallyl cyanoacetate, and esters thereof with hydroxyethyl acrylate or hydroxyethyl methacrylate.

Such malonic acid derivative having the general formula (3) may be produced by the conventional processes, one of which will be described by way of example. One part by weight of p-toluene sulfonic acid as a catalyzer is added to a mixture of 100 parts of cyanoacetic acid, 100 parts of allyl alcohol, 50 parts of benzene and 50 parts of cyclohexane. The mixture undergoes esterification at 70°-80° C. for about 24 hours while refluxing by means of a phase separator to remove water. After reaction, it is cooled, rinsed, dehydrated and distilled to remove the solvents. Thereafter it is further distilled under reduced pressure of 10 mmHg. The derivative aimed at is obtained by collecting the fraction at 110°-112° C.

The acrylic esters used in this invention are esters of acrylic acid with alkyl alcohols, β-ethoxy ethyl alcohol or β-methoxy ethyl alcohol.

In the copolymerization according to this invention, the amount of the malonic acid derivative having the general formula (2) or (3) is preferably 2-10% by weight, and more preferably 2-6%, relative to the acrylic ester. If it were less than 2%, the addition of malonic acid derivative would not have a sufficient effect, whereas for more than 10% the curing rate would be much higher and the tensile strength would increase owing to over-cure, but the hardness would increase, thus resulting in lower elongation and elasticity.

The reaction temperature for copolymerization is 5°-95° C., preferably 50°-70° C., and the reaction time is preferably 30 to 120 minutes.

As vulcanizing agents used for the acrylic rubber produced according to the present invention, tetramethylthiuram disulfide and tetraethylthiuram disulfide are preferable. Also, tetramethylthiuram monosulfide or thiazole is preferably used as a vulcanizing accelerator.

The acrylic rubbers produced according to the present invention show much higher curing rate and marked plateau effect in comparison with the conventional acrylic rubber cured with amines, as will be seen in FIG. 1 wherein (A) is the vulcanization curve for the acrylic rubber produced according to this invention and (B) is the curve for conventional acrylic rubber. The present acrylic rubbers also retain resistance to heat, oil, ozone, weathering and bend-cracking which the conventional acrylic rubber has. Furthermore, they have additional advantages of better processibility with a mixing roll, freedom from scorch during processing or storage, no corrosiveness to a curing mold and easy adhesion to metal inserts. Besides the present acrylic rubbers allow use of white carbon in the silica or talc series as well as the conventional carbon black as a reinforcing agent. This provides greater flexibility for production of colored rubber.

The acrylic rubber produced according to the present invention may be formed into rolls, seals, gaskets, "O"-rings, hoses and so on.

The following examples are included merely to aid in the understanding of the present invention. Unless otherwise stated, quantities are expressed as parts by weight.

EXAMPLE 1

A. In a flask were put 200 parts of water, 0.5 part of sodium laurylsulfate and 2 parts of polyoxyethylene lauryl ether as emulsifiers, 5 parts of allyl ethyl malonate, 0.05 part of potassium persulfate as polymerization initiator and 0.05 part of sodium hydrogen bisulfite as redox catalyst. The mixture was heated to 50°-70° C. while blowing nitrogen gas thereinto and 95 parts of ethyl acrylate was added drop by drop, taking 30 to 40 minutes, for emulsion polymerization to give vulcanizable acrylic rubber.

B. To 100 parts of the acrylic rubber thus prepared in (A) were added 50 parts of MEF (medium extrusion furnace) carbon, 1 part of stearic acid, 2.4 parts of tetramethylthiuram disulfide and 3.3 parts of dibenzothiazolyl disulfide. After kneading well in an open roll, the mixture was put into a curing mold and heated at 170° C. for 10 minutes. The rubber slab thus made was subjected to post cure at 150° C. for 16 hours. Table 1 summarizes the physical properties of the cured acrylic rubber in an original test, an air heat aging test at 150° C. for 70 hours and oil resistance tests, respectively.

                  Table 1                                                          ______________________________________                                                                              Volume                                                Hard-  Tensile   Elongation                                                                             change                                    Properties  ness   strength  percentage                                                                             percentage                                Kind of Test                                                                             (in HS)  (in kg/cm.sup.2)                                                                         (in %)  (in %)                                    ______________________________________                                         Original test                                                                            74       123       250     --                                        Air heat aging                                                                           78       142       215     --                                        test at 150° C.                                                         for 70 hours                                                                   Oil resistance                                                                 test                                                                           with JIS No. 1                                                                           78       131       270     --                                        oil at 150° C.                                                          for 70 hours                                                                   with JIS No. 3                                                                           69       118       310     --                                        oil at 150° C.                                                          for 70 hours                                                                   ______________________________________                                          (JIS is an abbreviation of the Japanese Industrial Standard.)            

EXAMPLE 2

Except that 4 parts of allyl methyl malonate and 96 parts of ethyl acrylate were used, the same mixing ratio and reaction conditions as in Example 1 were used to prepare vulcanizable acrylic rubber.

To 100 parts of the acrylic rubber were added 50 parts of white carbon, 1 part of stearic acid, 2.4 parts of tetraethylthiuram disulfide and 3.3 parts of dibenzothiazolyl disulfide. After kneading, the mixture was pre-cured at 170° C. for 10 minutes and post-cured at 150° C. for 4 hours. Table 2 shows the physical properties of the cured acrylic rubber measured as in Example 1.

                  Table 2                                                          ______________________________________                                                                              Volume                                                       Tensile   Elongation                                                                             change                                              Hardness strength  percentage                                                                             percentage                                Kind of test                                                                             (in HS)  (in kg/cm.sup.2)                                                                         (in %)  (in %)                                    ______________________________________                                         Original test                                                                            74       115       260     --                                        Air heat aging                                                                           79       135       270     --                                        test at 150° C.                                                         for 70 hours                                                                   Oil resistance                                                                 test                                                                           with JIS No. 1                                                                           78       137       230     -0.8                                      oil at 150° C.                                                          for 70 hours                                                                   with JIS No. 3                                                                           70       110       330     +11.7                                     oil at 150° C.                                                          for 70 hours                                                                   ______________________________________                                    

EXAMPLE 3

Except that 5 parts of allyl ethyl malonate, 80 parts of ethyl acrylate and 15 parts of butyl acrylate were used as monomers, the same mixing ratio and reaction contitions as in Example 1 were used. The acrylic rubber thus made was cured in the same manner as in Example 1 except that the post cure time was 7 hours. Table 3 shows the physical properties of the cured acrylic rubber.

                  Table 3                                                          ______________________________________                                                                              Volume                                                       Tensile   Elongation                                                                             change                                    Properties                                                                               Hardness strength  percentage                                                                             percentage                                Kind of Test                                                                             (in HS)  (in kg/cm.sup.2)                                                                         (in %)  (in %)                                    ______________________________________                                         Original test                                                                            65       109       300     --                                        Air heat aging                                                                           74       121       240     --                                        test at 150° C.                                                         for 70 hours                                                                   Oil resistance                                                                 test                                                                           with JIS No. 1                                                                           76       118       250     +2.3                                      oil at 150° C.                                                          for 70 hours                                                                   with JIS No. 3                                                                           58       102       380     +19.3                                     oil at 150° C.                                                          for 70 hours                                                                   ______________________________________                                    

EXAMPLE 4

Except that 8 parts of allyl ethyl malonate and 92 parts of ethyl acrylate were used, the same mixing ratio and reaction conditions as in Example 1 were used to produce vulcanizable acrylic rubber. It was then cured as in Example 1 except that tetraethylthiuram disulfide was used instead of tetramethylthiuram disulfide. Table 4 shows the physical properties of the cured acrylic rubber.

                  Table 4                                                          ______________________________________                                                                              Volume                                                       Tensile   Elongation                                                                             change                                    Properties                                                                               Hardness strength  percentage                                                                             percentage                                Kind of Test                                                                             (in HS)  (in kg/cm.sup.2)                                                                         (in %)  (in %)                                    ______________________________________                                         Original test                                                                            79       152       180     --                                        Air heat aging                                                                           84       168       150     --                                        test at 150° C.                                                         for 70 hours                                                                   Oil resistance                                                                 test                                                                           with JIS No. 1                                                                           83       162       190     -0.9                                      oil at 150° C.                                                          for 70 hours                                                                   with JIS No. 3                                                                           73       147       279     +11.8                                     oil at 150° C.                                                          for 70 hours                                                                   ______________________________________                                    

EXAMPLE 5

A. In a flask were put 200 parts of water, 0.5 part of sodium laurylsulfate and 2 parts of polyoxyethylene lauryl ether, 5 parts of allyl cyanoacetate, 0.05 part of potassium persulfate and 0.05 part of sodium hydrogen bisulfite. The mixture was heated to 50°-70° C. while blowing nitrogen gas thereinto and 95 parts of ethyl acrylate was added drop by drop, taking 30 to 40 minutes, for emulsion polymerization to give vulcanizable acrylic rubber.

B. To 100 parts of the acrylic rubber thus prepared were added 50 parts of MEF carbon, 1 part of stearic acid, 2 parts of tetramethylthiuram disulfide and 2 parts of dibenzothiazolyl disulfide. After kneading well in an open roll, the mixture was put into a curing mold and heated at 170° C. for 10 minutes. The rubber slab thus made was subjected to post cure at 150° C. for 16 hours. Table 5 shows the physical properties of the cured acrylic rubber.

                  Table 5                                                          ______________________________________                                                                              Volume                                                       Tensile   Elongation                                                                             change                                    Properties                                                                               Hardness strength  percentage                                                                             percentage                                Kind of Test                                                                             (in HS)  (in kg/cm.sup.2)                                                                         (in %)  (in %)                                    ______________________________________                                         Original test                                                                            75       145       260     --                                        Air heat aging                                                                           79       142       226     --                                        test at 150° C.                                                         for 70 hours                                                                   Oil resistance                                                                 test                                                                           with JIS No. 1                                                                           78       138       273     -0.9                                      oil at 150° C.                                                          for 70 hours                                                                   with JIS No. 3                                                                           67       122       310     +12.1                                     oil at 150° C.                                                          for 70 hours                                                                   ______________________________________                                    

EXAMPLE 6

Except that 7 parts of allyl cyanoacetate, 15 parts of methyl acrylate and 78 parts of ethyl acrylate were used as monomers, the same mixing ratio and reaction conditions as in Example 5 were used.

To 100 parts of the acrylic rubber thus prepared were added 50 parts of white carbon as a reinforcing agent, 1 part of stearic acid, 3 parts of tetraethylthiuram disulfide and 3 parts of dibenzothiazolyl disulfide. The acrylic rubber was cured as in Example 5 except that the post cure time was 4 hours.

EXAMPLE 7

Except that 5 parts of allyl cyanoacetate, 10 parts of acrylonitrile and 85 parts of butyl acrylate were used, the same mixing ratio and reaction conditions as in Example 5 were used.

The acrylic rubber thus prepared was cured as in Example 5 except that 1.5 parts of dibenzothiazolyl disulfide were used and the post cure time was 7 hours. Table 6 shows the physical properties of the cured acrylic rubber.

                  Table 6                                                          ______________________________________                                                            Tensile   Elongation                                                                             Volume                                    Properties                                                                               Hardness strength  percentage                                                                             change                                    Kind of Test                                                                             (in HS)  (in kg/cm.sup.2)                                                                         %)      (in %)                                    ______________________________________                                         Original test                                                                            70       135       210     --                                        Air heat aging                                                                           75       162       206     --                                        test at 150° C.                                                         for 70 hours                                                                   Oil resistance                                                                 test                                                                           with JIS No. 1                                                                           70       135       241     +0.5                                      oil at 150° C.                                                          for 70 hours                                                                   with JIS No. 3                                                                           61       112       255     +18.4                                     oil at 150° C.                                                          for 70 hours                                                                   ______________________________________                                    

EXAMPLE 8

Except that 7 parts of allyl cyanoacetate and 93 parts of ethyl acrylate were used as monomers, the same mixing ratio and reaction conditions as in Example 5 were used.

To 100 parts of the acrylic rubber thus prepared were added 50 parts of MEF carbon, 1 part of stearic acid, 2 parts of tetraethylthiuram disulfide and 2 parts of dibenzothiazolyl disulfide. The acrylic rubber was cured as in Example 5 except that the post cure time was 4 hours.

EXAMPLE 9

The vulcanizable acrylic rubber prepared in step (A) of Example 5 was cured at 170° C. by use of such vulcanizing agent, accelerators and retarder as shown in Table 7.

                  Table 7                                                          ______________________________________                                         Test No.       1         2         3                                           ______________________________________                                         Acrylic rubber 100       100       100                                                        parts     parts     parts                                       MEF carbon     50        50        50                                          Stearic acid   1         1         1                                           Tetramethylthiuram                                                                            2         2         2                                           disulfide                                                                      Dibenzothiazolyl                                                                              2         2         2                                           disulfide                                                                      Tetramethylthiuram                                                                            0.5       --        0                                           monosulfide                                                                    N-phenyl-β-                                                                              --        --        2                                           naphthylamine                                                                  (retarder)                                                                     ______________________________________                                    

FIG. 2 shows the vulcanization curves for these three tests. These test results show that the acrylic rubber produced according to the present invention has a large advantage over the conventional acrylic rubber in that the rise or start of vulcanization is adjustable by using a vulcanization accelerator or retarder. 

What are claimed are:
 1. A process for producing vulcanized acrylic rubber which comprises (1) copolymerizing (A) at least one acrylic ester selected from the group consisting of β-methoxyethylacrylate, β-ethoxyethylacrylate and an ester of acrylic acid with an alkyl alcohol with (B) 2-10% by weight, based on the weight of the acrylic ester component, of a malonic acid derivative having an active methylene group of the formula ##STR5## wherein R₁ represents vinyl, allyl or methallyl, and X represents cyano or COOR₂ in which R₂ represents methyl, ethyl or propyl, in an aqueous medium and in the presence of a copolymerization catalyst at a temperature of 5°-95° C. for 30-120 minutes, the acrylic ester component being added to the copolymerization system in a dropwise manner, and (2) vulcanizing the resultant copolymer with a vulcanizing agent of the thiuram series.
 2. The process according to claim 1 wherein said malonic acid derivative is used in an amount of 2-6% by weight based on the weight of the acrylic ester component.
 3. The process according to claim 1 wherein the temperature of copolymerization is 50°-70° C.
 4. The process according to claim 1 wherein the vulcanizing agent is tetramethylthiuram disulfide or tetraethyl thiuram disulfide.
 5. The process according to claim 1 wherein said malonic acid derivative is allyl ethyl malonate.
 6. The process according to claim 1 wherein said malonic acid derivative is allyl methyl malonate.
 7. The process according to claim 1 wherein said malonic acid derivative is allyl cyanoacetate.
 8. The process according to claim 1 wherein said malonic acid derivative is methallyl cyanoacetate.
 9. The process according to claim 1 wherein said malonic acid derivative is vinyl cyanoacetate. 